Electrodynamic sound pickup or sound-track cutting device

ABSTRACT

An electrodynamic sound pickup which also functions as a soundtrack cutting device, two angularly disposed preferably stereophonic recordings of an engraved sound track are sensed by a phonograph needle mounted at the free end of a needle arm oscillating about its other end and carrying two transducer coils having their effective coil range disposed in a magnetic air gap formed by two spaced pole shoes opposing each other. The winding planes of the coils are arranged transversely with respect to the longitudinal direction of the needle arm, and one of the two pole shoes is split at its end adjacent the air gap accommodating both coils, so as to form a pair of pole-shoe fingers each extending through the free central space of each of the pair of coils wherein the effective coil portions disposed within the air gap extend along an arc with a center angle not exceeding 90*.

limited tates Patent 1 1111 3,729,596

Tth 1 Apr. 24, 1973 [54] ELECTRODYNAMIC SOUND PICKUP Primary ExaminerRaymond F. Cardillo, Jr.

OR SOUND-TRACK CUTTING DEVICE [75] Inventor: Mikls Tth, Lubeck, Germany [73] Assignee: Hermann Wilms, Lubeck, Germany [22] Filed: Apr. 27, 1971 [21] Appl. No.: 137,817

[30] Foreign Application Priority Data Apr. 29, I970 Germany ..P 20 20 903.4

52 us. C1. ..179/100.41 K [51] 1nt.Cl. ..H04r 9/16 [58] Field of Search ..179/10().41 K, 100.41 D

[56] References Cited UNITED STATES PATENTS 2,027,168 l/l936 Harrison ..l79/l00.4l D 3,142,729 7/l964 Davis ..l79/l00.4l K 2,114,471 4/1938 Keller et al. v.l79/l0().4l K

FOREIGN PATENTS OR APPLlCATlONS Great Britain ..l79/100.41 K

Attorney-Karl F. Ross [57] ABSTRACT An electrodynamic sound pickup which also functions as a sound-track cutting device, two angularly disposed preferably stereophonic recordings of an engraved sound track are sensed by a phonograph needle mounted at the free end ofa needle arm oscillating about its other end and carrying two transducer coils having their effective coil range disposed in a magnetic air gap formed by two spaced pole shoes opposing each other. The winding planes of the coils are arranged transversely with respect to the longitudinal direction of the needle arm, and one of the two pole shoes is split at its end adjacent the air gap accommodating both coils, so as to form a pair of pole-shoe fingers each extending through the free central space of each of the pair of coils wherein the effective coil portions disposed within the air gap extend along an arc with a center angle not exceeding 90.

5 Claims, 2 Drawing Figures Patented April 24, 1973 7 3,729,596

2 Sheets-Sheet l 'INVENTORJ Miklo's To'fh Attorney Patented April 24, 1973 3,729,596

2 Sheets-Sheet 2 INVENTOR. Miklds Tdfh Attorney ELECTRODYNAMIC SOUND PICKUP OR SOUND- TRACK CUTTING DEVICE DESCRIPTION OF THE PRIOR ART In a known sound pickup, the coils, which are arranged normally to each other and within a magnetic field, are supported by a needle arm mounted with its inner end in such a way that the center of oscillation is disposed at the mounting end, while the coils are spaced therefrom by a certain distance, and are con- 1 nected to the needle arm by a supporting body made of insulating material. This construction allows a relatively large amplitude of movement for the coils, whereby, at a defined number of windings, a relatively great number of lines of flux may be intersected. This is desirable because of a high signal or transducer voltage output. But, of course, such a construction is afflicted with the drawback that the moment of inertia of the oscillating system is large, since the path of movement of the coils include a large radius. In addition, a relatively wide air gap must be provided for the safe functioning, the wide gap causing a low field intensity in the air gap.

Attempts have been made to overcome this disadvantage by mounting the coils on a magnetically permeable armature arranged on the needle arm, thereby reducing the effective width of the air gap. However, as the armature oscillates in unison with the system, reproduction distortions are encountered, since the armature constantly changes the lines of flux within the air gap. Also a construction of this type does not represent a genuine electrodynamic system.

Other prior dynamic sound pickups are known, in which the two transducer coils are shaped triangularly, and are arranged in parallel relation side by side at both sides of the needle arm, adapted to oscillate in any direction, wherein the coil sections effective in energy transducing extend in parallel relation to the needle arm. This type of construction requires a wide and thus disadvantageous air gap, since the effective coil sections disposed within the air gap are arranged one behind another.

Another known embodiment is constructed in such a way that the planes of the two coils mounted on the needle arm are disposed normal to one another within an air gap of a permanent magnet. In order to allow, at a defined width of the air gap, a large number 'of windings, it is not possible to place the windings side by side in such a construction, but instead they must be superposed, resulting in correspondingly large heights of the coils. This in turn requires an expensive mounting of the high coils on the needle arm, in order to avoid a subsequent oscillation, or a mechanical fading out of the coil.

Besides the direct mounting of the transducer coils on the needle arm, it is also possible to connect the coils by separate arms of a coupling member to the needle arm and to suspend them from elastic strips secured above the sound pickup in any suitable manner. In this case the axes of the coils are disposed in parallel relation relative to the associated air gap in which the effective coil section oscillates. Since in this construction the coils are spaced in great distance relative to the needle arm, and since the parts required for holding the coils, form an inert mass, these sound pickup systems are afflicted with a frequency-dependent inertia which is disadvantageous with respect to the frequency characteristic of the sound pickup.

OBJECT OF THE INVENTION The object of this invention resides substantially in avoiding the above mentioned disadvantages by providing an electrodynamic sound pickup the construction and oscillating system of which provides a frequency 0 characteristic of the transducer system affected to minimum by dynamic masses and of a compact construction.

SUMMARY OF THE INVENTION further that one of the two pole shoes is split at its end adjacent the air gap for both coils so as to form a pair of pole shoe fingers in such a manner that each one of the pole shoe fingers extends into an through the free central space of each one of the coils, wherein the effective coil portion ranges of each coil extend along an are having a center angle of One of the pair of pole shoes arranged normally relative to each other includes at its end a semicircular recess defining the one end of the air gap, wherein the effective coil portion ranges disposed in the air gaps, and the surfaces of the pole shoe fingers extending through the coils and defining the other side of the air gaps extend in parallel relation to each other. For example, both coils are arranged symmetrically with respect to an imaginary vertical plane extending through the needle arm. Suitably the portions of the coils adjacent the needle arm are embedded in a mass of plastic material or adhesive completely surroundings the needle arm.

The end of the needle arm disposed opposite to the phonograph needle is secured in a known manner to one of the pole shoes and includes a weakened portion serving as resilient support for the needle arm allowing it to oscillate in any direction, wherein the weakened portion, as compared with the diameter of the needle arm, is formed by a thin wire-like part of metal or plastic material and this weakened portion is surrounded by an elastic cushion.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified diagrammatic side view of the sound pickup system according to the invention; and

FIG. 2 is a part front view of the sound pickup system according to FIG. 1 in enlarged scale.

DESCRIPTION OF THE PREFERRED EMBODIMENT The magnet system of the sound pickup includes a permanent magnet 1 to the ends of which a first pole shoe 2 and a bridge or yoke 3 are attached. The second pole shoe 4 extends in parallel relation to the magnet l and is secured to the yoke 3. The materials suitable for these members are well known.

The air gap 5 for the pair of transducer coils 6 and 7' is formed at the spaced opposing ends of the pair of pole shoes 2 and 4. The effective ranges 6a and 7a of the coils 6 and 7 are disposed within the air gap 5, while their opposite portions 6b and 7b adjacent the needle arm 8 are embedded in a mass of plastic material or adhesive 9, serving as retaining means, and completely surrounding the needle arm 8 carrying the phonograph needle 10. 7

As is evident from FIG. 2, the coils 6 and '7 in the illustrated exemplified embodiment, which are disposed transversely with respect to the longitudinal direction of the needle arm 8, have substantially the form of a trapezoid, the longer base lines 6a and 7a of which extend in parallel relation to the shape of the lower semicircular recess of the pole shoe 2 defining the air' gap at one side, while the zone of the shorter base lines 6b and 7b of the coils are secured to the needle arm by means of the rigid mass 9. Both transducer coils are disposed homologously symmetrically with respect to an imaginary vertical plane 1 1 extending through the needle arm 8.

The pole shoe 4 disposed normally with respect to the pole shoe 2 has, in order to form two pole shoe fingers 12 and 13 a split end, adjacent the air gap 5 for the pair of coils 6 and 7, wherein these pole shoe fingers 12 and 13 are formed as annular segments and are disposed homologously symmetrically to each other at either side of the imaginary plane 11, one pole shoe finger 12 extending into and through the coil 6 while the other pole shoe finger 13 extends into and through the coil 7. The effective coil portion ranges (in and 7a disposed within the air gap 5 extend, as already mentioned, in parallel relation to the semicircular recess 2a of the pole shoe 2 defining the one ends of the air gap 5, and also in parallel relation to the surfaces 12a and 13a of the pole shoe fingers 12 and 13 extending through the coils 6 and 7 and defining the other ends of the air gaps 5.

In practice the construction of the pole shoe 4 with its finger-like split end has been found to be particularly advantageous with respect to economical production technique; but evidently this pole shoe 4 could be replaced by two pole shoes extending in parallel relation to each other, and through the central free spaces of the coils, in the same manner as already described in connection with the split pole shoe fingers.

The needle arm 8 is secured to the pole shoe 4'by means of a brass sleeve inserted into a bore 14 of the pole shoe. The sleeve 15 is secured against axial shifting within the bore 14 by a bolt 16.

The needle arm 8 may, in a manner known per se, oscillate in any direction about a weakened portion which is formed, in comparison with the diameter of the needle arm, by a relatively thin wire-like part 17 of metal or plastic material, connecting the sleeve 15 and the needle arm 8 proper which latter is formed by a tubular body. In practice it has been found to be advantageous to form the part 17 as a threadlike element of plastic material because, if a metal wire is used, mechanical resonances may be generated in the audible frequency range.

In the zone of weakening, or the connecting thread 17 respectively, a resilient cushion 18 of rubber surrounding the same is provided, disposed between a contacting surface 19 of the pole shoe 4 and the retaining means 9 surrounding the needle arm 8. This retaining means is connected to a plate 20 of plastic material or hard paper disposed at the side facing the cushion 18 and surrounding the needle arm 8. The rubber cushion 18 is clamped between the contacting surface 19 and the plate 20 under a defined pretension, with the result that centering forces may be applied to the oscillating system in a simple manner. For reasons of stability the retaining body 9 for the coils 6 and 7 may be encased completely in a stable layer of paper. in order to save weight, the lower part of the body 9 is formed substantially conical while its upper part, within which the coils are embedded, has a substantially rectangular cross sectional configuration.

Soldering contacts 21 are provided at the lower extension of the yoke 3 to which the ends of the coil wires must be soldered in a common manner. The signal voltages are then derived from the contact pins 22 and fed, for example, to a signal voltage amplifier. The conductor extending through the yoke 3 must, of course, be insulated.

if, for example, one starts out from the fact that the signals to be sensed have been engraved in the sound track under an angle of the oscillations of the needle arm 8 about the weakened portion take place in two planes including an angle. The planes have been indicated in FIG. 2 in phantom lines 23 and 24.

It is evident that when the needle arm 8 oscillates in the plane 23 a voltage will be induced within the coil 6, since then its entire effective part range 6a intersects the flux lines within the air gap 5 synonymously. It is true that, in this direction of oscillation, also the coil 7 will intersect flux lines, but within it two oppositely directed equal voltages are induced which will cancel each other. Conversely, this holds true also, if oscillations take place in the plane 24, in which case only the coil 7 will produce signal voltages.

It will be evident from FIGS. 1 and 2 that due to the fact that the coils 6 and 7, or their planes of windings respectively, are aligned to each other and are disposed transversely to the longitudinal direction of the needle arm' 8, and that the pole shoe fingers 12 and 13 extend through the central free spaces of these coils, a very low and compact construction of the sound pickup is possible. The air gap 5 may be kept very narrow since, according to the described mode of construction, the coils may be made relatively wide and flat for obtaining a great number of windings. In addition this includes the advantage that the radii of oscillations of the coils are relatively small.

Important is also the dimensioning of the arcuately shaped effective coil portion ranges 6a and 7a disposed in the air gap 5. in order to fully exploit the possible dimensions of the air gap, extending in FIG. 2 about the above-mentioned coil portion ranges of each coil should extend along an arc the center angle of which amounting preferably at a maximum to 90, or substantially 90 respectively. The center of this angle is in this case disposed at the intersection of the two lines 23 and 24. If a center angle smaller than 90 is selected, which is practically possible, the signal voltage will naturally be lower, since then the effective coil portion ranges 6a and 7a are correspondingly shorter and a lesser voltage will be induced therein. In case the arcuate coil portions 6a and 7a would extend along an arc with a center angle greater than 90", then this would be afflicted with the disadvantage that, on the one hand, the coils 6 and 7 with their lower ends (FIG. 2) would be disposed in the plane of the phonograph needle 10, and that, on the other hand, asymmetrical relationship would result causing, for example, that upon moving the needle arm 8 in the plane 24 the voltages induced in coil 6 would not cancel each other.

Finally it should be pointed out that the above described construction may also be used as a sound track cutting device, wherein the phonograph needle 10 must be replaced by an engraving stylus serving to cut groove-like sound tracks. The signals or cutting voltages are in this case supplied to coils 6 and 7 via the terminals 22 in a known manner.

What is claimed is:

1. An electrodynamic transducer, comprising:

a permanent-magnet yoke having a pair of spacedapart poles defining an air gap between them, one of said poles being formed with a pair of codirectionally extending parallel but transversely spaced fingers of generally arc-segmental cross section, the other pole being formed with a semicircular recess overhanging and spaced from said fingers;

a stylus angularly oscillatable in said recess and generally extending along the axis thereof;

an elongated spring anchored at one end to said one of said poles and extending generally axially and parallel to said fingers while carrying said stylus; and

a pair of generally quadrant-shaped coplanar coils lying in a common plane transverse to said axis and secured, within a region overhung by said fingers to said stylus, said coils each surrounding a respective finger and having an arc-segmental effective winding portion extending through at most of are between the respective finger and the wall of said recess.

2. The electrodynamic transducer defined in claim 1 wherein said coils and said fingers are symmetrical with respect to an imaginary axial plane of said recess through said stylus, said coils having generally trapezoidal configuration with small bases proximal to said stylus and large bases forming said effective winding portions.

3. The electrodynamictransducer defined in claim 2, further comprising a mass of plastic material on said stylus, said small bases being embedded in said mass, said spring being a thin-metal member surrounded by an elastic cushion.

4. The electrodynamic transducer defined in claim 3 wherein said thin metal member is anchored to said mass and said transducer is provided with coil-retaining means confronting said cushion and provided with a stiff sheet surrounding the portion of said stylus proximal to said mass.

5. The electrodynamic transducer defined in claim 4 wherein said cushion is an elastic rubber body under pretension between said one of said poles and said sheet. 

1. An electrodynamic transducer, comprising: a permanent-magnet yoke having a pair of spaced-apart poles defining an air gap between them, one of said poles being formed with a pair of codirectionally extending parallel but transversely spaced fingers of generally arc-segmental crosssection, the other pole being formed with a semicircular recess overhanging and spaced from said fingers; a stylus angularly oscillatable in said recess and generally extending along the axis thereof; an elongated spring anchored at one end to said one of said poles and extending generally axially and parallel to said fingers while carrying said stylus; and a pair of generally quadrant-shAped coplanar coils lying in a common plane transverse to said axis and secured, within a region overhung by said fingers to said stylus, said coils each surrounding a respective finger and having an arc-segmental effective winding portion extending through at most 90* of arc between the respective finger and the wall of said recess.
 2. The electrodynamic transducer defined in claim 1 wherein said coils and said fingers are symmetrical with respect to an imaginary axial plane of said recess through said stylus, said coils having generally trapezoidal configuration with small bases proximal to said stylus and large bases forming said effective winding portions.
 3. The electrodynamic transducer defined in claim 2, further comprising a mass of plastic material on said stylus, said small bases being embedded in said mass, said spring being a thin-metal member surrounded by an elastic cushion.
 4. The electrodynamic transducer defined in claim 3 wherein said thin metal member is anchored to said mass and said transducer is provided with coil-retaining means confronting said cushion and provided with a stiff sheet surrounding the portion of said stylus proximal to said mass.
 5. The electrodynamic transducer defined in claim 4 wherein said cushion is an elastic rubber body under pretension between said one of said poles and said sheet. 